Method of utilizing a wide frequency range for signaling channels



April 28, 1931. E. 1. GREEN 1,802,420

METHOD OF UTILIZING A WIDE FREQUENCY RANGE FOR SIGNALING CHANNELS FiledJan. 30, 1950 2 Sheets-Sheet 1 fir fb INVENTOR ZZZ Gneen ATTORNEY E. I.GREEN 1,802,420 METHQD OI UTILIZING A WIDE FREQUENCY RANGE FOR SIGNALINGCHANNELS April 28, 193}.

Filed Jan. 50, 1930 2 Sheets-$heet 2 rlllL m miwm fi Vaz'ce CircuitINVENTOR ZZZ. Gneen/ ATTQRNEY Vbe'ce Circuit Patented Apr. 28, 1931"entries stares PATENT FFIE ESTILL I. GREEN, OF EAST ORANGE, NEW JERSEY,ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OFNEW YORK METHOD OF UTILIZING A WIDE FREQUENCY RANGE FOR SIGNALINGCHANNELS Application filed January 30, 1930. Serial No. 424,676.

This invention relates to transmission systems capable of transmittingsuch a wide range of frequencies that a large number of signalingchannels may be carved out of the available range by employing differentcarrier frequencies. 7

It has been proposed that a circuit comprising two coaxial conductors beutilized for the transmission of a Wide range of frem quencies, and thatthis range be employed either for television purposes or for ordinarytelephone or telegraph channels. 1V here such a wide range offrequencies is to be divided up among a number of signaling channels theproblem of selecting between the difi erent channels at the terminalsbecomes one of considerable difficulty because of the high frequenciesinvolved.

Various terminal arrangements for selecting between the channels havebeen proposed. It has been proposed, for example, to select each channelfrom the other channels at high frequency and regardless of its positionin the frequency spectrum by means of I 'ilter individual to thechannel, as is done in ordinary carrier telephony over open wire lines.another scheme is to divide tne channcl nto groups occupying aconsiderably larger frequency range than a single channel and at thereceiving terminal select a wanted group from all of the other groups bymeans of a high frequency filter. The selected group is then steppeddown in the frequency range to a part of the spectrum as near zerofrequency as possible and then the individual channels in the group areselected from each other at low frequency, the signals being finallydetected from the channels after selection.

As will be shown later, thecfirst of these methods is very wasteful ofthe available frequency spectrum by reason of the fact that for channelshigh in tne frequency spectrum a greater separation or dead space mustbe left between channels in order to obtain suffioient discriminationnear the filter cut-offs than is required for channels low in thefrequency spectrum. The second of the two methods enables the channelsin each group 5% to be closely spaced regardless of the location wantedband after demodulation.

of the group in the frequency spectrum because the ultimate selection asbetween channels occurs at low frequency.

Even with the second method, however, there is a considerable waste. offrequency space for separating the different groups because theselection of the group occurs at high frequency, with the consequencethat the filter cut-offs are not sharp enough to permit the neighboringchannels of adjacent groups to be brought very close together.

In accordance with the present invention it is proposed to overcome thisdifliculty by using the high frequency filters merely to roughly selecta given group, permitting par tially suppressed channels from adjacentgroups to bepassed through the filter along the sides of the filtercharacteristic lying without the nominal cut-ofi point. lhe chan nelsthus selected are demodulated by beating with a carrier frequency sochosen that the partially suppressed and unwanted channels along thesides of the cut-off characteristic will not lap over upon the channelsof the By selecting the channels of the desired group after suchdem'odulating operation the relatively sharp characteristics of the lowfrequency filters of the individual channels will suppress the alreadypartially suppressed, and demodulated channels corresponding to theunwanted group. This method of selection enables the adjacent channelsof two adjacent groups to be approximately as closely spaced as adjacentchannels in the same group.

In order to prevent the unwanted and partially suppressed channels fromlapping over into the range of the wanted group after the first'demodulating operation it is necessary that the beating carrierfrequency be some distance below the lower nominal cut-off frequency ofthe high frequency filter. 1 F or example, if the beating frequency islocated below the lower nominal cut-off point of the high frequencyfilter at the point where the sloping side of the-filter characteristicproduces a suflicient suppression to reduce transmission below thecrosstalk limit, the lowest channel of the wanted group will, afterdemodulation, be above zero frequency in the frequency spectrum by afrequency range equal to the frequency difference between the beatingfrequency and the nominal cut-oil 1 1: m m point of tne high irequencyInter. ihis necessarily involves a somewhat greater separation betweenthe individual channels of the group because the low frequency selectionbetween channels takes place at a somewhat higher point in the frequencyspectrum.

In orderto, bring the wanted channels down closer to zero frequency forfinal selection, the present invention involves,.as a furtherrefinement, the location of the beating frequency for the firstdemodulating operation at a point substantially half-way between thelower npminal cut-off point of the high fre-.

i be located only half as hi h above zero frequency as in the casepreviously discussed, the individual channels in the group may besomewhat more closely spaced than in'the previous case.

The invention will now be more fully understood from the followingdescription when read in connection with the accompanying drawings,Figures 1, 2 and 3 of which are curves illustrating the principles ofthe in vention and Fig. 4 of which is a schematic circuit arrangementembodying the invention.

In order to better visualize the invention, let us compare two types ofterminal arrangements for selecting channels, the first type involvingindividual channel selection at high frequency with a receiving bandfilter or selector for each channel, the filters of all of the channelsbeing connected together, and the second type of terminal arrangementinvolving dividing the entire frequency range into groups of channelswith high frequency filters for selecting a desired group at highfrequency, the selected group then being brought by demodulationapproximately to the bottom of the frequency spectrum, whereupon theindividual channels are selected by low frequency filters, after whichthe individual signals may be detected from each channel.

Before proceeding with the analysis of these two schemes, it will-bedesirable to consider a general principle and a formula in connectionwith the division of frequency ranges. Nith present technique it may besaid in general that whenever it is desired to separate two bands orgroups of frequencies by means of filters a certain unused frequencyinterval or dead space must be left between the two bands in order toobtain sufficient discrimination near the filter cut-offs. The amount ofthis dead space may be specified in terms of the nominal cut-offfrequencies of these filters. t is more convenient, however, andprobably sufficiently correct for This last formula is a very usefulinstrument in the study of methods of separating frequency bands.

Let us now proceed to analyze the two methods of handling av wide rangeof frequencies at the terminals. Let it be assumed for concreteness thata clear frequency range of 1,000,000 cycles is available fortransmission in each direction, this range being exclusive of spacenecessary for the filters which separate the opposite directions oftransmission. If it be assumed that the upper range can be reduced infrequency or that separate circuits will be used for trans mission inopposite directions, it is sufiicient to consioer only the range fromapproximately 0 to 1,000,000 cycles which is available for one directionof transmission. The problem is to determine what will happen if each ofthe different methods is applied in connection with this frequencyrange.

Consider first the band filter or single modulation method. In the firstplace, it may be noted that with present filter technique the separationspace necessary between band filters for bands transmitted in the samedirection is roughly .06 times the maximum frequency of the lower band.Let us assume that the necessary frequency width in each channel is2,500 cycles. The value of the minimum frequency m may be taken as 3,500cycles. Although the precise values assumed may be open to somequestion, they are of ion value in giving the problem a quantitativebasis. Substituting these values in Formula (at), we find that thenumber of channels derivable by the band filter method from a totalfrequency range of 1,000,000 cycles is only 54. This means that thefrequency range is being used at an efficiency of only 54 2500 1,000,000

or 13 per cent.

l V e now come to the second scheme in which double modulation isemployed. It will be recalled that the principle embodied in this secondscheme is that of dividing a complete range (for' one direction oftransmission) into groups and that these groups after being brought nearto the bottom of the frequency scale are divided by means of theindividual channel band filters. It is not essential to this scheme thatthe number of channels be the same for each group. It is convenient,however, to assume that this is the case. Fortunately, the maximumnumber of channels obtainable is not appreciably effected by thisassumption. If the number of channels to be put in each group is known,the frequency band required for the group may be determined by FormulaThe number of groups that can be placed in a range of 1,000,000 cyclesmay then be determined with the aid of the same formula. Considering theinterval between groups, however, there may be some question as towhether the selectivity fraction should not be modified. One reason forthis is that the space used for separation between groups at theterminalsmay also be used in dividing up the ran e for amplificationrepeater points. Accordingly, it is of interest to assume, first, thatsufficient space between groups is obtained when 8 06, and second, thatthe value of 8 should be .12.

As has been noted the total number of groups can be found if the numberof chan nels per group is known. The product of the two which is thetotal number of channels available, will vary according to the number ofchannels per group and will reach a maxim m for a particular value ofchannels per group. By successive computations for diffcrent number ofchannels per group, the optimal. value is found. 1V hen s has a value of.06 for both the grouping, and the channel filters, appears that thenumber of channels is a maximum when about 10 channels are assigned toeach group. The width of each --oup about 35.000 cycles, the number ofgroups is 17, and the total number of channel bands is 170. When 5 12for the. grouping ir' erval, the most favorable number of channels pergroup is 18, the width of each group is about 83,000 cycles, the numberof groups 8, and the total number of channels is 144. The frequencyrange is used at an efficiency of 42 per cent. in the former case and 36per cent. in the latter case. Thus, we have increased the efficiencyover that of scheme (1) by about or per cent.

In order to obtain the foregoing spacing of the channels, it has beenassumed that the band constituting a desired group of channels iscompletely selected by the first selecting operation at high frequencybefore the first demodulation. In order to obtain the closest spacingbetween individual channels of the same group, it has been assumed thatthe carrier or beating frequency for the first demodlating step islocated at the lower nominal cut-oii' poi it of the high frequency groupselecting filter, so that the lowest channel of the desired group willafter demodulation be as close as possible to zero frequency.

Such an arrangement is illustrated by the curves of Fig. 1 where theheavy line curve A represents the attenuation characteristic of a highfrequency filter having nominal cut-off points at f and f the filterserving to select channels 1, 2, 8, 4t and 5 of a group from the nearestadjacent channels, such as 5 and 1, of two adjacent groups. Theindividual channels are represented by the filter char cteristics of thecorresponding low frequency filters which are to be ultimately used toseparate the individual channels of a group after first beingdemodulated to the lower part of the frequency spectrum. lVith respectto the characteristic A of the high frequency filter for selecting thedesired group, and also with respect to the characteristics (shown inheavy dotted lines) of the high frequency filters for selecting theadjacent groups, it will be noted that the sloping sides outside thenominal cut-off points are relatively flat because the. selection takesplace at high frequency. Consequently, in the case of the filtercharacteristic A, the nearest channel 5 of the group immediately belowthe desired group must be spaced such distance away from the cut-offfrequency f that any frequencies of the channel 5 pass ing through thefilter A will be below the crosstalk limit.

Assuming that the crosstalk limit on the lower sloping side of thefilter A is at the frequency f and that the carrier frequency fordemodulation be located at frequency f it will be evident that if anyindividualchannels such as indicated in dotted lines, are included inthe range between f and f such channels will be lapped over into therange of the channels 1 to 5 inclusive, as aresult of the firstdemodulating operation which brings the channel 1 as nearly as possibleto the zero frequency range. In order to prevent this overlapping ofunwanted frequencies from adjacent. groups into the range of the'desired group after demodulation, it is necessary to provide a wastedfrequency space, such as the range f to f between the desired group andthe nearest group below.

A somewhat wider spacing f to f will'be required'between the desiredgroup and the nearest group above, this wider separation being due tothe fact that at this point the selection takes place higher in thefrequency spectrum.

By having the beating frequency located at the lower cut-off point ofthe filter A the desired channels of the group will, after demodulation,be located as close as possible to thezero frequency point, with theresult that the channels of the group will have the closest possiblefrequency separation. It will also be noted from the filter curves of Ithe channels 1 to 5 inclusive that the spacing between the channelsincreases as the channels go up in the frequency range. This spacing isdetermined, however, by the posi ion of the channel in the frequencyrange after the first demodulation, and hence is much closer than ispossible between adjacent groups at high frequency. Further more, theslopin sides of the filter characteristics of the channels 1 to 5inclusive are much more nearly vertical than in the case of the highfrequency filter A, as these characteristics represent thecharacteristics of the low frequency filters after the channels havebeen stepped down by demodulation.

In accordance with the present invention i is proposed to reduce theseparation between groups, as shown in Fig. 1, at the expense of aslightly greater separation be tween the individual channels of thegroup. Thisis accomplished by depending upon the high frequency filterto effect only a rough selection of the desired group, thus permittingchannels of adjacent groups to be par-.

tially transmitted along the sides of the filter characteristic, andthen stepping down the group by a properly chosen beating fre quency, sothat after demodulation the partially suppressed channels of theadjacent groups will be completely eliminated by the selectivity of theindividual channel filters which are effective at relatively low frequencies.

In order to accomplish this result the location of the beating frequencywith respect to the nominal cut-0E of the group band filter isimportant. Fig. 2 shows the principle underlying one arrangement inaccordance with the invention. rrere the characteristic of a highfrequency band filter is shown at A, this band-filter passing at highfrequencies the groupof channels designated 1 to 5, inclusive. Thesloping sides of the'filter characteristic A partially pass channelssuch as 3, 4: and 5' of an adjacent group which, in this case, iscrowded much more closely to the nominal cutoff frequency f of thefilter whose char acteristic is shown at A. The suppression for channelsin the adjacent group increases with the distance of the channel fromthe chosen group, sothat the frequencies falling within channel 3 aresuppressed much more than the frequencies of channel 5, for example.

Instead of locating the beating frequency at the lower nominal cut-offpoint f of the group filter as in the case of Fig. 1, the carrierfrequency may be located at f,,, which is the point at which the slopingside of the filter characteristic suppresses the transmission below theallowable crosstalk limit. 6 sequei when the various frequencies passedby the group selee'ing filter are de- 7 ed by he beating frequency noneof the unwanted frequencies between 7'", and f wil lap over onto thedesired channels 1 to 5, inclusive. After demodulation the channelfrequency corresponding to 7, will he stepped dow proximately to zero,and consequently the riequency f corresponding to the lower nominalcut-off point of the high frequency group filter will he stepped down toa frequency equal to f instead of to zero frecy, as in t 1e case ofFig. 1. It follows, ore, that channel 1, instead of being ed down to apoint as near as possible to ency, is somewhat above zero frei rh theresult that the desired chanilusive, must be spaced somefurther apartthan in the case of Fig. l to allow for the fact that the low frequencyselection takes place somewhat higher in the frequency spectrum.Notwithstanding that the width of the group is greater than in the caseof F 1, a large number of channels may be carved out of the availablefrequency spectrum dueto the fact that the channels of the adjacentgroups can be permitted to approach along the sides. of the filtercharacteristic A toward the nominal cutoff points f and f No appreciableinterchannel interference results from this due to the fact that the lowfrequency filters corresponding to 1 to 5, inclusive,will, by theirrelatively sharp characteristics for low frequency selecion, effectivelyeliminate the frequencies of channels of adjacent groups which fall witlin the sloping sides of the high frequency filter characteristic.

The outstanding advantage of the arrangement just described is that thefrequency separation or dead space between groups need not be as greatas in the arrangement of Fig. 1. V hile,as shown in Fig. 2, neighboringchannels of adjacent groups are spaced somewhat further apart thanadjacent channels within a given group, this is not necessary in orderto obtain the desired selectivity and is merely so shown in thisinstance in order to make more clear from ob servation of the diagramthe arrangement of the channels in groups. In actual practiceneighboring channels of adjacent groups may be substantially as closelyspaced as neighboring channels within the group.

The disadvantage of the arrangement is that the beating frequency cannotbe at the lower nominal cut-0E point of the group filter as in Fig. 1,but must be some distance be low as otherwise there will be someoverlapping of the partially suppressed unwanted channels of the roupbelow and the desired channels which lie between the nominal cutofipoints of the filter. This limitation on the location of the beatingfrequency renders it impossible to step the group down as low in thefrequency spectrum for final selection as in the case of ig. 1, andhence the individual channels of the selected group must be spacedsomewhat further apart. An alternative arrangement which permits asclose spacing between groups as in the case of Fig. 2 but which permitsof somewhat closer spacing of th channels within a group, is shown inFig. 3.

In this figure the beating frequency, instead of being located at fwhere the suppression of the filter whose characteristic is shown at Ais suificient to bring the transmission below the allowable crosstalklimit, is located at f about half-way between the frequuency 7, and thelower nominal cut-off frequency f of the filter. After demodulation thefrequencies between 7. and f overlap upon the frequencies between f andf so that in effect the unwanted channels lying along the side of thefilter characteristic between 7",, and f are doubled or folded over uponeach other without overlapping into the range of the desired channels 1to 5, inclusive. After demodulation channel 1 will only be spaced abovezero by an amount approximating f f instead of by an amountapproximating fi-fg, as in Fig. 2. Selection of individual channels ofthe desired group will, therefore,take place somewhat lower in thefrequency spectrum in the case of Fig. 3 than in the case of Fig. 2,permitting of somewhat closer spacing of the channels in each group. Asbefore, therelatively sharp characteristics of the individualchannelfilters operating at'low frequency are effective to suppress anychannels of adjacent groups which were partially transmitted due to theupper and lower sloping sides of the high frequency filtercharacteristic A.

It will be obvious from a comparison of Figs. 1, 2 and 3 that thearrangements of Figs. 2 and 3 reduce the dead space between groups by avery large factor. On the basis of the same assumptions as were used indetermining the number of. channels available in a given frequency-spaceby employing the arrangement of Fig. 1, it is estimated that the numberof channels obtainable from a band of 1,000,000 cycles by using thearrangement of Fig. 3 will be about 245 as compared with 170 for thearrangement of Fig. 1. The efficiency with which the frequency range isutilized is about 61 per cent. for the arrangement of F i 3 as comparedwith about 42 per cent. for the arrangement of Fig. 1.

Fig. 4-is a diagram of a possible arrangement of terminal apparatus forapplying the method of Fig. 3 to a coaxial conductor system whichpermits transmission. up to 1,000,000 cycles. For convenience it hasbeen assumed that four-wire operation will be used, with a separatecoaxial circuit for each direction of transmission. This, of course, isnot essential to the scheme. It is also assumed that the channels willbe arranged in groups of five channels each and that only two stepsdemodulation or modulation (one for each group and one for each channel)will be employed. In all cases a frequency interval of .06 times thenominal cut-off frequency is assumed necessary to attain the desiredsuppression of unwanted frequencies. The scheme provides 49 groups offive channels each, or a total of 245 channels, in a frequency band fromabout 3.5 to 990 kilocycles. The corresponding frequency allocations aregiven in the following table:

Low ire quency position Location of group carrier High frequencyBy'increasing the number of channels in each group it would be possibleto effect a substantial reduction in the number of grouping filters andgroup demodulators or modulators, but this would be obtained at theexincrease in the apparatus required. In the extreme case in which onlyone channel might be assigned to a group, a total of two demodulators ormodulators and two filters would be needed per channel at each terminal.This compares with a requirement of about 1.2- demodulators ormodulators and 1.2 filters per channel when groups of five channels eachare used. It is thought that the assignment of from five to ten channelsto each group represents a reasonable compromise between the desire toavoid too great complexity of apparatus and the desire to obtain themaximum number of channels.

The circuit arrangement shown in Fig. 4 is readily understood. Voicecurrents transmitted over an ordinary low frequency line TL, forexample, pass from a hybrid coil into a channel modulator CM which issupplied with a carrier frequency from a source CO. The desired sideband(or bands) is passed through a channel band filter CBF and is combinedwith similar channel bands from other channel-band filters of the group.The bands corresponding to the group of channels are then applied to agroup modulator GM which may include amplifying apparatus), the groupmodulator being supplied with a carrier frequency from a source GO forthe purpose of stepping the group of channels up in the frequencyspectrum. The stepped-up channels are then passed through the groupfilter GF and applied with groups from other group filters to a commoncircuit, such as a coaxial conductor line OXL of any, known type.Intermediate repeaters may be provided in the line at one or morepoints, as shown at R, for amplifying the frequencies transmitted overthe line. The several groups received from the distant end of thecoaxial conducting system CXL are then selected by the group filter,such as The group of channels selected by a given high frequency orgroup filter is then applied to a group demodulator GD supplied with abeating fre quency from a source G0, the beating frequency being locatedwith respect to the channels of the group, as shown in Fig. 3, so thatthe channels of the group may be stepped down as near to zero frequencyin the frequency spectrum as is possible without causing interlappingwith neighboring channels of an adjacent group. The particular channelwhich is desired is, after demodulation, selected by a channel bandfilter, such as CBF', and the final signal band is detected by asuitable channel demodulator CD which may, if desired, be supplied. withacarrier frequency from a source CO corresponding to, the originalcarrier from the source CO at the sending end. A low pass filter LF ispro-.

vided to suppress harmonics and high frequencies from the desired signalband which is then impressed upon the outgoing voice circuit TL.

For transmission in the opposite direction, thatlis from TL to TL, thetransmission passes through a channel modulator shown in the upper righthand corner of the diagram and thence through other transmittingapparatus similar to that already described in connection with the lineTL. After passing over another coaxial conducting arrangement similar tothat illustrated, the channel will pass through receiving apparatussimilar to that associated with the receiving end of the coaxialconductor CXL, this receiving apparatus including the final channeldemodulator and low pass filter, as illustrated in the upper left handcorner of the diagram, the band passed by the low pass filter beingapplied through the hybrid coil to the voice circuit TL.

It will be noted that the lower group of channels need not he stepped upin the frequency spectrum by a second modulating step and hence thegroup modulating apparatus, and if desired, the group filter at thesending end, need not be provided. It follows, of course, that the groupdemodulator and if desired the group filter at the receiving end, willbe unnecessary.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely difierent from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists in roughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the selecting devicetogether with partially suppressed channels of adjacent groups lyingwithin the range between the nominal cut-off point and the point atwhich the selecting device substantially extinguishes unwantedfrequencies, and stepping down the desired group of channels in thefrequency spectrum by demodulating them with a beating frequency locatedsuch distance away from the nominal cut-off point of the selectingdevice that frequency components corresponding to the partiallysuppressed channels of the adjacent group will not after demodulationlap over into the range of the stepped down channels of the desiredgroup.

2. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists in roughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the selecting devicetogether with partially suppressed channels of adjacent groups'lyingwithin the range between the nominal cut-off point and the point atwhich the selecting device substantially extinguishes unwantedfrequencies, stepping down the desired group of channels in thefrequency spectrum by demodulating them with a beating frequency located such distance away from the nominal cut-off point of the selectingdevice that frequency components corresponding to the partiallysuppressed channels of the adj acent group will not after demodulationlap over into the range of the stepped-down chan nels of the desiredgroup, and selecting the channels of the desired group at theirstepped-down position in the frequency spectrum while substantiallyextinguishing the partially suppressed channels of the adjacent grouppassed as the result of the high frequency selecting operation.

3. In a system in which a large number of transmission channels arearranged in groups along the frequency s Jectrum, the m thod ofreceiving which consists in roughly selecting at high frequency adesired group falling within the nominal cut-off point of the se lectingdevice together with partially suppressed channels of adjacent groupslying within the range between the nominal cut-off point and the pointat which the selecting device substantially extinguishes unwantedfrequencies, stepping down the desired group of channels in thefrequency spectrum by demodulating them with a beating frequency locatedsuch distance away from the nominal cut-off point of the selectingdevice that frequency components corresponding to the partiallysuppressed channels of the adjacent group will not after demodulationlap over into the range of the stepped-down channels 7 of the desiredgroup, selecting the channels of the desired group at their stepped-downposition in the frequency spectrum while substantially extinguishing thepartially suppressed channels of the adjacent group passed as the resultof the high frequency selecting operation, and detecting the signal fromeach channel after suchselection.

4:. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists inroughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the selecting devicetogether with partially suppressed channels of adjacent groups lyingwithin the range between the nominal cutoff point and the point at whichthe selecting device substantially entinguishes unwanted frequen cies,and stepping down the desired group of channels in the frequencyspectrum by emodulating them with a beating frequency locatedsubstantially midway between the nominal cut-off point of the selectingdevice and the point at which it substantially extingnishes unwantedfrequencies, whereby frequency components corresponding to the partiallysuppressed channels of the adjacent group will lap over each other afterdemodulation without overlapping the range of the steppeddown channelsof the desired' group.

5. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists in roughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the select ing devicetogether with partially suppressed channels of adjacent groups lyingwithin the range between the nominal cut-off point and the point atwhich the selecting device substantially extinguishes unwantedfrequencies,-

stepping down the desired group of channels in the frequency spectrum bydemodulating them with a beating frequency located substantially midwaybetween the nominal cutoff point of the selecting device and the pointat which it substantially extinguishers unwanted frequencies, wherebyfrequency com ponents corresponding to the partially suppressed channelsof the adjacent group will lap over each other after demodulationwithout overlapping the range of the stepped-' down channels of thedesired group, and selecting the channels of the desired group at theirstepped-down position in the frequency spectrum while substantiallyextinguishing the partially suppressed channels of the adj a cent grouppassed as the result of the high frequency selecting operation.

6. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists in roughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the selecting devicetogether with partially suppressed channels of adjacent groups lyingwithin the rangebetween the nominal cut-ofi point and the pointat whichthe selecting device substantially extinguishes unwanted frequencies,stepping down the desired group of channels in the frequency spectrum bydemodulating them with a beating frequency located substantially midwaybetween the nominal cutoff point of the selecting device and the pointat which it substantially extinguishes unwanted frequencies, wherebyfrequency components corresponding to the partially suppressed channelsof the adjacent group will lap over each other after demodulation without overlapping the range of the steppeddown channels of the desiredgroup, selecting the channels of the desired group at their stepped-downposition in the frequency spec trum while substantially extinguishingthe partially suppressed channels of the adjacent group passed as theresult of the high frequency selecting operation, and detecting theselectin g at high vice and the point signal from each channel aftersuch selection.

7. In a system in which a large number of transmission frequencies arearranged in groups along the frequency spectrum, the method of r ceivingwhich consists in roughly frequency a desired group falling within thenominal cut-off point of the sel cting device, together with partiallysuppressed frequencies of adjacent groups lying within the range betweenthe nominal cut-off point of the highfrequency selecting device and thepoint at which this device substantially extinguishes unwantedfrequencies, stepping down the selected frequencies by demodulating themwith a beating frequency located at such a point that the partiallysuppressed frequencies of at least one of the adjacent groups willoverlap one another after demodulation without overlapping the desiredgroup of frequencies.

8. The method of allocating frequencies for transmission over a mediumcapable of handling a wide frequency range, which consists in arrangingthe transmission frequencies in groups along the frequency spectrum, andseparating these groups by such an interval that the high frequencyselection of a desired group only partially suppresses frequencies ofadjacent groups, the said interval between groups being so determined,however, that these partially suppressed frequencies are substantiallyextinguished in the process of low frequency selection subsequent todemodulation with a beating frequency located at such a point that thepartially suppressed frequencies of at least one of the adjacent groupsoverlap one another after demodulation.

9. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists in roughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the selecting devicetogether with partially suppressed channels of adjacent groups lyingwithin the range between the nominal cut-off point and the point atwhich the selecting device substantially extinguishes unwantedfrequencies, and stepping down thedesired group of channels in thefrequency spectrum by demodulating them with a beating frequency locatedbetween the nominal cut-off point of the selecting dewhich itsubstantially extinguishes unwanted frequencies, whereby frequencycomponents corresponding to .the partially suppressed channels of theadjacent group will lap over each other after demodulation withoutoverlapping the range of the stepped-down channels of the desired group.a

10. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the

method of receiving which consists in roughly selecting at highfrequency a desired group falling within the nominal cut oi'l' point ofthe selecting device together with partially suppressed channels ofadjacent groups lying within the range between the nominal cut-off pointand the point at which the selecting device substantially extinguishesunwanted frequencies, stepping down the desired group of channels in thefrequency spectrum by demodulating them with a beating frequency locatedbetween the nominal cut-off point of the selecting device and the pointat which it substantially extinguishes unwanted frequencies, wherebyfrequency components corresponding to the partially suppressed channelsof the adjacent group will lap over each other after demodulationwithout overlapping the range of the stepped-down channels of thedesired group, and selecting the channels of the desired group at theirstepped-down position in the frequency spectrum while substantiallyextinguishing the partially suppressed channels of the adjacent grouppassed as the result of the high frequency selecting operation.

11. In a system in which a large number of transmission channels arearranged in groups along the frequency spectrum, the method of receivingwhich consists in roughly selecting at high frequency a desired groupfalling within the nominal cut-off point of the selecting devicetogether with partially suppressed channels of adjacent groups lyingwithin the range between the nominal cut-off point and the point atwhich the selecting device substantially eXtinguishes unwantedfrequencies, stepping down the desired group of channels in thefrequency spectrum by demodulating them with a beating frequency locatedbetween the nominal cut-off point of the selecting device and the pointat which it substantially extinguishes unwanted frequencies, wherebyfrequency components corresponding to the partially suppressed channelsof the adjacent group will lap over each other after demodulationwithout overlapping the range of the stepped-down channels of thedesired group, selecting the channels of the desired group at theirstepped-clown position in the frequency spectrum while substantiallyextinguishing the partially suppressed channels of the adjacent grouppassed as the result of the high frequency selecting operation, anddetecting the signal from each channel after such selection.

In testimony whereof, I have signed my name to this specification this29th day of January, 1930.

ESTILL I. GREEN.

